Those skilled in the art are well aware of numerous applications in which an output current or voltage must be monotonically modulated. Examples of such applications include, without limitation, light emitting diode (LED) dimming circuits and motor control circuits.
FIG. 1 illustrates an example of a LED dimming circuit 10. The circuit 10 is configured to generate a drive current Id which is applied to a string of LEDs 12. The circuit includes a current mirror circuit 14 having an input circuit path configured to receive a variably controlled current Iv and an output circuit path configured to output the drive current Id at a drive node. The current mirror circuit 14 is formed by a pair of MOSFET transistors 16 and 18. The source terminals of the transistors 16 and 18 are connected to a reference supply node 20 and the gate terminals of the transistors 16 and 18 are coupled together. The source-drain path of transistor 16 defines the input circuit path of the current mirror circuit 14 configured to receive the variable current Iv, and the source-drain path of transistor 18 defines the output circuit path of the current mirror circuit 14 coupled to the drive node and configured to supply the drive current Id. A differential amplifier 22 is provided to control the accuracy of the current mirroring operation performed by the mirror circuit 14. The non-inverting input (+) of the amplifier 22 is connected to the drain terminal of the transistor 16 and the inverting input (−) of the amplifier 22 is connected to the drain terminal of the transistor 18. The output of the amplifier 22 is connected to the commonly-connected gate terminals of the transistors 16 and 18.
The variably controlled current Iv is generated by a current source circuit 30. The current source circuit 30 receives a reference current Iref and generates the variably controlled current Iv as a function of the reference current Iref and a control input Cv (wherein the control input Cv specifies a proportional functional relationship between Iref and Iv that depends on input Cv). The variable current Iv thus varies in response to changes in the control input Cv, and as a result, through the current mirroring function, the drive current Id also varies in response to the input Cv. This variation in the drive current Id in response to change in the control input Cv causes a variation in the light output from the LED string 12. By varying the control input Cv, a dimming of the light produced by the LED string 12 is provided.
It is important in the dimming operation for a monotonic relationship to exist between the change in the control input Cv and the change in the variably controlled current Iv. There is accordingly a need in the art for a current source circuit capable of ensuring delivery of such a monotonic relationship.